In recent years, a reduction in fuel consumption of automobiles has been increasingly desired along with a demand for energy conservation. Research and development have been conducted in order to deal with such a demand. For example, a reduction in rolling resistance has been desired for tires. The rolling resistance of tires may be reduced by various methods such as optimizing the tire structure and the tire material. In particular, a reduction in rolling resistance has been studied from the viewpoint of the tire material (i.e., a rubber composition that exhibits low heat build-up).
As a rubber composition that exhibits low heat build-up, a rubber composition that contains a modified rubber and silica or carbon black as a filler has been developed, for example. For example, a modified rubber obtained by modifying a polymerization active end of a conjugated diene polymer obtained by anionic polymerization using an organolithium compound with an alkoxysilane derivative having a functional group that interacts with a filler has been proposed.
However, most modified rubbers are obtained by modifying a polymer of which the polymer end exhibits living properties. Specifically, only a small number of modified rubbers obtained by modifying cis-1,4-polybutadiene that is particularly important as a material for a tire sidewall rubber or a tire tread rubber have been proposed. Moreover, a rubber composition that contains a modified rubber obtained by modifying cis-1,4-polybutadiene and a filler such as silica or carbon black does not exhibit a sufficient modification effect. Specifically, the heat build-up of the resulting rubber composition is not sufficiently reduced. In particular, a modified rubber used for a rubber composition containing carbon black exhibits a modification effect to only a small extent.
In order to deal with the above-mentioned problems, a method that utilizes an end-modified conjugated diene polymer obtained by reacting an active end of a conjugated diene polymer having a high cis content obtained using a rare earth catalyst with a functional group-containing alkoxysilane derivative that interacts with a filler, and a method that utilizes a modified rubber obtained by adding a condensation catalyst to the reaction system when performing alkoxysilane modification have been proposed (see Patent Documents 1 and 2).    Patent Document 1: WO 03/046020    Patent Document 2: JP-A-2005-8870